Plasmids and transposons acquired by Salmonella typhi in man

Salmonella typhi, in a patient with enteric fever, acquired two plasmids that together conferred resistance to the drugs used to treat the illness. One of them, of IncHI, conferred chloramphenicol and sulfonamide resistance and altered the phage type of the S. typhi. An indistinguishable plasmid was present in a strain of Klebsiella aerogenes probably carried by the patient since before the start of therapy. The other plasmid, belonging to no known Inc group, carried a trimethoprim-resistance transposon, indistinguishable from Tn7. The klebsiella, believed to have been the source of the chloramphenicol-resistance plasmid, carried the Tn7-like transposon but on a different plasmid from that in the S. typhi. All the resistance genes acquired by the S. typhi could thus have come, directly or indirectly, from the klebsiella. Both R plasmids from the klebsiella, though compatible with one another, determined H-type pili and resistance to potassium tellurite; their interrelationship is discussed.     

Incidence of resistance to ampicillin,chloramphenicol, kanamycin and tetracycline amongSalmonella species isolated in The Netherlands in 1972, 1973 and 1974

The resistance ofSalmonellae to drugs has been studied in the Netherlands since 1958. In 1972, 1973, and 1974 respectively, 14241, 13086, and 22927 strains were tested for resistance to ampicillin, chloramphenicol, kanamyci and tetracycline. From 1973 all strains were also tested for resistance to trimethoprim. In the period covered, the yearly incidence of resistance to at least one of the above drugs ranged from 39.2% to 45.6% of all strains obtained from various sources (humans, animals, animal products, sewage, etc.). A new finding in the period 1972 to 1974 was that many multiply resistant strains emerged inS. typhimurium and inS. dublin isolated from calves and cattle. In 1974, 64.4% of all strains ofS. typhimurium from these animals appeared to be resistant to ampicillin, tetracycline, chloramphenicol and kanamycin, and 25.5% of those ofS. dublin were found to be resistant to chloramphenicol and tetracycline. Of all strains ofSalmonellae examined in 1973 and 1974 respectively, 0.15 and 0.22% were resistant to trimethoprim, the main component of the twin-drug cotrimoxazol. Of the 142 strains ofS. typhi isolated in 1972 to 1974 two were resistant to tetracycline only, and one was resistant to all four antibiotics. The others had a normal susceptibility pattern.     

Identification of a Novel G2073A Mutation in 23S rRNA in Amphenicol-Selected Mutants of Campylobacter jejuni

Objectives

This study was conducted to examine the development and molecular mechanisms of amphenicol resistance in Campylobacter jejuni by using in vitro selection with chloramphenicol and florfenicol. The impact of the resistance development on growth rates was also determined using in vitro culture.

Methods

Chloramphenicol and florfenicol were used as selection agents to perform in vitro stepwise selection. Mutants resistant to the selective agents were obtained from the selection process. The mutant strains were compared with the parent strain for changes in MICs and growth rates. The 23S rRNA gene and the L4 and L22 ribosomal protein genes in the mutant strains and the parent strain were amplified and sequenced to identify potential resistance-associated mutations.

Results

C. jejuni strains that were highly resistant to chloramphenicol and florfenicol were obtained from in vitro selection. A novel G2073A mutation in all three copies of the 23S rRNA gene was identified in all the resistant mutants examined, which showed resistance to both chloramphenicol and florfenicol. In addition, all the mutants selected by chloramphenicol also exhibited the G74D modification in ribosomal protein L4, which was previously shown to confer a low-level erythromycin resistance in Campylobacter species. The mutants selected by florfenicol did not have the G74D mutation in L4. Notably, the amphenicol-resistant mutants also exhibited reduced susceptibility to erythromycin, suggesting that the selection resulted in cross resistance to macrolides.

Conclusions

This study identifies a novel point mutation (G2073A) in 23S rRNA in amphenicol-selected mutants of C. jejuni. Development of amphenicol resistance in Campylobacter likely incurs a fitness cost as the mutant strains showed slower growth rates in antibiotic-free media.     

Incidence of resistance to chloramphenicol and tetracyclines among 13502Salmonella strains isolated in 1961

Summary The rate of resistance to tetracycline and chloramphenicol amongSalmonella strains isolated in the Netherlands in 1961 was found to be 3.96%, the corresponding figures for 1958/1959 and 1961 being 2.08 and 1.29 respectively. In this country the total number ofSalmonella types found to develop resistance to either tetracycline or chloramphenicol now amounts to 38. Almost 77% of all resistant strains isolated in 1961 were found among the human pathogenS.typhimurium. The relative frequency of resistance in this organism was 8.18%, as compared with 2.50% in 1958/1959 and 1.80% in 1960. In 1961 some cross infections caused by tetracycline resistant strains ofS.typhimurium were observed in man and on one occasion also in a herd of calves. A similar outbreak due to a tetracycline resistant strain ofS.bovis morbificans was seen in a hospital. As almost 87% of all antibiotic resistant strains found in 1961 originated from human patients, the resistance must be largely attributed to the therapeutic use of the drugs in question.     

Naturally occurring antibiotic resistance inBacillus sphaericus andBacillus licheniformis

Bacillus sphaericus is an aerobic, spore-forming, gram-variable bacillus. Certain strains of this organism are extremely pathogenic for mosquito larvae. Strains from four different serotypes ofB. sphaericus were found to be naturally resistant to streptomycin and to chloramphenicol. Both entomocidal and nonentomocidal strains of this organism exhibited similar patterns of resistance to these antimicrobial agents. In contrast, other members of the genusBacillus, includingB. subtilis, B. thuringiensis, B. laterosporus, andB. amyloliquefaciens, proved to be quite sensitive to these antibiotics. Four strains ofB. licheniformis were found to be resistant to chloramphenicol but sensitive to streptomycin.     

Incidence of resistance to tetracycline,chloramphenicol and ampicillin amongSalmonella species isolated in The Netherlands in 1965 and 1966

In 1965 and 1966 respectively 9628 and 10467Salmonella strains were screened for resistance to tetracycline and chloramphenicol. In 1965 the major half of the strains were also tested for ampicillin resistance and in 1966 all strains passed this test. In 1965 20.9% and in 1966 21.7% were found resistant, mainly to tetracycline. The incidence of resistance to chloramphenicol remained low (less than 1%). By far the majority of the resistant strains belonged toS. typhi murium andS. panama. The rate of resistance in the former organism remained nearly equal to that observed in 1963 – 1964, whereas inS. panama it has diminished.Ampicillin-resistant strains were found in as many as 23 different types ofSalmonella. In 8 types in which ampicillin resistance was found, only animal strains were concerned. These findings may represent cases of primary ampicillin resistance. In the human strains ofSalmonella the incidence of ampicillin resistance increased from 11.6% in 1965 to 15.3% in 1966.     

Resistance of Salmonellae Isolated in 1959 and 1960 to Tetracyclines and Chloramphenicol

The resistance of cultures of Salmonella typhimurium to tetracyclines and chloramphenicol has been examined periodically. Although none of 200 cultures isolated prior to 1948 was resistant to the antibiotics, 5% of 100 cultures from man and 9% of 100 cultures from fowls which were isolated in 1956 and 1957 were resistant to tetracyclines. Among 158 cultures isolated from man and 100 cultures isolated from fowls in 1959 and 1960, 13.9 and 29%, respectively, were resistant to tetracyclines. In the last series, cultures resistant to chloramphenicol were found for the first time. Among 150 cultures of other Salmonella serotypes from man and 137 similar cultures isolated from fowls in 1959 and 1960, 5.3 and 8%, respectively, were found resistant to tetracyclines. There is no obvious explanation for the higher percentage of resistant strains occurring in S. typhimurium than in other serotypes.     

Investigating the Molecular Mechanisms of Organophosphate and Pyrethroid Resistance in the Fall Armyworm Spodoptera frugiperda

The fall armyworm Spodoptera frugiperda is an economically important pest of small grain crops that occurs in all maize growing regions of the Americas. The intensive use of chemical pesticides for its control has led to the selection of resistant populations, however, to date, the molecular mechanisms underlying resistance have not been characterised. In this study the mechanisms involved in the resistance of two S. frugiperda strains collected in Brazil to chlorpyrifos (OP strain) or lambda-cyhalothrin (PYR strain) were investigated using molecular and genomic approaches. To examine the possible role of target-site insensitivity the genes encoding the organophosphate (acetylcholinesterase, AChE) and pyrethroid (voltage-gated sodium channel, VGSC) target-site proteins were PCR amplified. Sequencing of the S. frugiperda ace-1 gene identified several nucleotide changes in the OP strain when compared to a susceptible reference strain (SUS). These result in three amino acid substitutions, A201S, G227A and F290V, that have all been shown previously to confer organophosphate resistance in several other insect species. Sequencing of the gene encoding the VGSC in the PYR strain, identified mutations that result in three amino acid substitutions, T929I, L932F and L1014F, all of which have been shown previously to confer knockdown/super knockdown-type resistance in several arthropod species. To investigate the possible role of metabolic detoxification in the resistant phenotype of the OP and PYR stains all EST sequences available for S. frugiperda were used to design a gene-expression microarray. This was then used to compare gene expression in the resistant strains with the susceptible reference strain. Members of several gene families, previously implicated in metabolic resistance in other insects were found to be overexpressed in the resistant strains including glutathione S-transferases, cytochrome P450s and carboxylesterases. Taken together these results provide evidence that both target-site and metabolic mechanisms underlie the resistance of S. frugiperda to pyrethroids and organophosphates.     

Molecular Characterization of Irish Salmonella enterica Serotype Typhimurium: Detection of Class I Integrons and Assessment of Genetic Relationships by DNA Amplification Fingerprinting

Salmonella enterica is among the principal etiological agents of food-borne illness in humans. Increasing antimicrobial resistance in S. enterica is a cause for worldwide concern. There is concern at present in relation to the increasing incidence of human infection with antimicrobial agent-resistant strains of S. enterica serotype Typhimurium, in particular of phage type DT104. Integrons appear to play an important role in the dissemination of antimicrobial resistance genes in many Enterobacteriaceae including S. enterica. In this study the antimicrobial susceptibilities and phage types of 74 randomly collected strains of S. enterica serotype Typhimurium from the Cork region of southern Ireland, obtained from human, animal (clinical), and food sources, were determined. Each strain was examined for integrons and typed by DNA amplification fingerprinting (DAF). Phage type DT104 predominated (n = 48). Phage types DT104b (n = 3), -193 (n = 9), -195 (n = 6), -208 (n = 3), -204a (n = 2), PT U302 (n = 1), and two nontypeable strains accounted for the remainder. All S. enterica serotype Typhimurium DT104 strains were resistant to ampicillin, chloramphenicol, streptomycin, Sulfonamide Duplex, and tetracycline, and one strain was additionally resistant to trimethoprim. All DT104 strains but one were of a uniform DAF type (designated DAF-I) and showed a uniform pattern of integrons (designated IP-I). The DT104b and PT U302 strains also exhibited the same resistance phenotype, and both had the DAF-I and IP-I patterns. The DAF-I pattern was also observed in a single DT193 strain in which no integrons were detectable. Greater diversity of antibiograms and DAF and IP patterns among non-DT104 phage types was observed. These data indicate a remarkable degree of homogeneity at a molecular level among contemporary isolates of S. enterica serotype Typhimurium DT104 from animal, human, and food sources in this region.     

Genetic and Molecular Properties of an Infectious Antibiotic Resistance (R) Factor Isolated from Klebsiella

A Klebsiella strain of human origin that was resistant to ampicillin, chloramphenicol, kanamycin, neomycin, streptomycin, and tetracycline was found to have all of these resistances associated with a R factor and a satellite molecular species of deoxyribonucleic acid (DNA) with an average buoyant density of 1.710 in cesium chloride gradients. There was no evidence of the existence of DNA with other buoyant densities. The strain bears two separable mutations for chloramphenicol resistance, both of which are associated with the R factor (KR9). Exposure of the Klebsiella strain to acridine derivatives or to ethidium bromide (which was more efficient) resulted in partial losses of resistance accompanied by the disappearance of the satellite DNA peak or shifts in its density. The R factor and its component genes were conjugally transmitted across generic boundaries and maintained in new hosts with different efficiencies. The basis of this difference lies not only in the efficiency of conjugal transfer but also in the stability of the components after transfer. All of the resistance genes and the resistance transfer factor were cotransducible by phage Plkc from Escherichia coli. Partially resistant strains could be reconstituted to full resistance or to a recombined pattern of partial resistance by conjugation with donors having complementary resistance patterns. This recombination serves as an efficient mechanism for rescuing superinfecting genes that are otherwise intracellularly excluded. KR9 is an fi⁺ type of R factor which in the natural state does not appear to be as repressed in conjugal transfer as other R factors.     

The Mechanism of Heterogeneous Beta-Lactam Resistance in MRSA: Key Role of the Stringent Stress Response

All methicillin resistant S. aureus (MRSA) strains carry an acquired genetic determinant – mecA or mecC - which encode for a low affinity penicillin binding protein –PBP2A or PBP2A′ – that can continue the catalysis of peptidoglycan transpeptidation in the presence of high concentrations of beta-lactam antibiotics which would inhibit the native PBPs normally involved with the synthesis of staphylococcal cell wall peptidoglycan. In contrast to this common genetic and biochemical mechanism carried by all MRSA strains, the level of beta-lactam antibiotic resistance shows a very wide strain to strain variation, the mechanism of which has remained poorly understood. The overwhelming majority of MRSA strains produce a unique – heterogeneous – phenotype in which the great majority of the bacteria exhibit very poor resistance often close to the MIC value of susceptible S. aureus strains. However, cultures of such heterogeneously resistant MRSA strains also contain subpopulations of bacteria with extremely high beta-lactam MIC values and the resistance level and frequency of the highly resistant cells in such strain is a characteristic of the particular MRSA clone. In the study described in this communication, we used a variety of experimental models to understand the mechanism of heterogeneous beta-lactam resistance. Methicillin-susceptible S. aureus (MSSA) that received the mecA determinant in the laboratory either on a plasmid or in the form of a chromosomal SCCmec cassette, generated heterogeneously resistant cultures and the highly resistant subpopulations that emerged in these models had increased levels of PBP2A and were composed of bacteria in which the stringent stress response was induced. Each of the major heterogeneously resistant clones of MRSA clinical isolates could be converted to express high level and homogeneous resistance if the growth medium contained an inducer of the stringent stress response.     

The in vitro Resistance to Antibiotics of Mycoplasma mycoides var. capri

S ummary . Resistance to dihydrostreptomycin, chloramphenicol and oleandomcin has been induced in vitro in a strain of Mycoplasma mycoides var. capri. The chloramphenicol resistant strain showed high resistance to oleandomycin, but the converse was not observed. The same cross resistance was also found, but to only a very small extent, in Staphylococcus aureus and Streptococcus faecalis. No cross resistance was found in three other species of bacteria examined. The osmotic lysis of the normal and the chloramphenicol resistant strains of M. mycoides var. capri were similar.     

A novel point mutation of acetylcholinesterase in a trichlorfon-resistant strain of the oriental fruit fly Bactrocera dorsalis (Diptera: Tephritidae)

Acetylcholinesterase (AChE) is the target enzyme of organophosphorus and carbamate insecticides. We applied trichlorfon to select resistant strains of Bactrocera dorsalis Hendel in the laboratory. Two trichlorfon-resistant strains, the Tri-R₁ strain with 18.23-fold resistance and the Tri-R₂ strain with 69.5-fold resistance, were obtained. Three known mutations, I159V, G433S and Q588R were identified in AChE of two resistant strains, and a novel mutation, G365A, was identified in the more resistant Tri-R₂ strain. The modeled 3-D-structure of AChE showed that G365A and G433S are closely adjacent in the gorge above the catalytic site S235. Mutations of G365A and G433S resulted in a steric hindrance by stronger Van der Waals force between two sites. Such a minor structural change might block insecticides from squeezing through the gorge to reach the active site, but not the natural substrate. Compared with the susceptible strain, the AChE activity of the Tri-R₁ strain and the Tri-R₂ strain was 0.87- and 0.67-fold, the K _m value of the Tri-R₁ strain and the Tri-R₂ strain was 0.11- and 0.10-fold, the V _max value of two resistant strains was 0.26- and 0.15-fold, whereas, the I ₅₀ to trichlorfon significantly increased by 9.07- and 13.19-fold. These results suggested that the novel point mutation G365A of AChE might be involved in increasing resistance to trichlorfon in the resistant strain of oriental fruit fly.     

An integrated in silico based subtractive genomics and reverse vaccinology approach for the identification of novel vaccine candidate and chimeric vaccine against XDR Salmonella typhi H58

Salmonella typhi is notorious for causing enteric fever which is also known as typhoid fever. It emerged as an extreme drug resistant strain that requires urgent attention to prevent its global spread. Statistically, about 11–17 million typhoid illnesses are reported worldwide annually. The only alternative approach for the control of this illness is proper vaccination. However, available typhoid vaccine has certain limitations such as poor long-term efficacy, and non-recommendation for below 6 years children, which opens the avenues for designing new vaccines to overcome such limitations. Computational-based reverse vaccinology along with subtractive genomics analysis is one of the robust approaches used for the prioritization of vaccine candidates through direct screening of genome sequence assemblies. In the current study, we have successfully designed a peptide-based novel antigen chimeric vaccine candidate against the XDR strain of S. typhi H58. The pipeline revealed four peptides from WP_001176621.1 i.e., peptidoglycan-associated lipoprotein Pal and two peptides from WP_000747548.1 i.e., OmpA family lipoprotein as promising target for the induction of immune response against S. typhi. The six epitopes from both proteins were found as immunogenic, antigenic, virulent, highly conserved, nontoxic, and non-allergenic among whole Salmonella H58 proteome. Furthermore, the binding interaction between a chimeric vaccine and human population alleles was unveiled through structure-based studies. So far, these proteins have never been characterized as vaccine targets against S. typhi. The current study proposed that construct V2 could be a significant vaccine candidate against S. typhi H58. However, to ascertain this, future experimental holistic studies are recommended as follow-up.     

Molecular characterization of antibiotic resistant Escherichia coli isolates recovered from food samples and outpatient Clinics,KSA

Multidrug-resistant Escherichia coli is one of the most important public health concern worldwide that can be transferred through the food of animal origin to human being causing serious infection. The genetic responsibility of such resistant genes (Plasmids, integrons, and transposons) can be easily transmitted from the resistant strain to another. Therefore, the main objectives of the study is the molecular characterization of the resistant Escherichia coli isolates recovered from food samples and human isolates collected from outpatient clinics, KSA especially the resistance strains against aminoglycoside resistance genes which are responsible for the resistance against gentamicin and the resistance caused β-lactamases genes. Examination of food samples revealed 120 Escherichia coli isolates (22.22%) (30 strains O26: K60, 28 strains O128: K67, 20 strains O111: K58, 18 strains O126: K58, 10 strains O55: K59, 9 strains O86: K61 and 5 strains O157: H7). All the strains were highly resistance to penicillin, amoxicillin-clavulanic and erythromycin with a percentage of 100%, while the resistance to gentamicin, ampicillin, oxytetracycline, chloramphenicol, norfloxacin, trimethoprim, and nalidixic acid were 83%, 75%, 65.3%, 55.8%, 36.5%, 30.7% and 26.9% respectively. On the other hand, 59.6% of tested strains were sensitive to ciprofloxacin. Positive amplification of 896?bp fragments specific for aacC2 genes were observed by PCR designated for the detection of the aminoglycoside resistance genes. Meanwhile, multiplex PCR designed to detect the ampicillin and amoxicillin-clavulanic acid resistant E. coli isolates revealed positive amplification of 516?bp fragments specific for BlaTEM gene with all the resistant strains to ampicillin and amoxicillin-clavulanic acid. Moreover, positive amplification of 392?bp fragments specific for BlaSHV resistant gene were observed with (60.52%) of E. coli isolate. While all the tested strains were negative for amplification of BlaOXA_1.     

Mechanism of Chloramphenicol Resistance in Staphylococcus epidermidis

The mechanism of chloramphenicol resistance in several multiple-resistant Staphylococcus epidermidis strains has been studied and shown to be due to the presence of the enzyme, chloramphenicol acetyltransferase. As with S. aureus, the inactivating enzyme in S. epidermidis appears to be the product of a structural gene on the chloramphenicol plasmid because resistance and enzyme activity are concurcurrently lost after growth in acridine orange or at elevated temperatures. The synthesis of chloramphenicol acetyltransferase in S. epidermidis has been compared with the function of a similar enzyme in chloramphenicol-resistant S. aureus with the conclusion that the kinetics of induction, products of the reaction, and general properties of the enzymes are identical. The chloramphenicol acetylating enzyme from S. epidermidis has been purified to a state of homogeneity and compared with the analogous purified S. aureus enzyme. Both purified preparations consist of native enzymes with molecular weights of 80,000, and evidence is presented that is consistent with their being made up of four identical subunits of 20,000 each. The two staphylococcal enzymes are identical with respect to pH optimum, apparent affinity (K_m) for chloramphenicol, heat denaturation, and immunological reactivity, but they differ in electrophoretic mobility, chromatographic behavior, substrate specificity, and sensitivity to inhibition by mercuric ion.     

Incidence of resistance to ampicillin,chloramphenicol, kanamycin,tetracycline and trimethoprim of Salmonella strains isolated in The Netherlands during 1975–1980

From 1975–1980, about 130 000 Salmonella strains isolated from various sources were tested for resistance to ampicillin, chloramphenicol, kanamycin, tetracycline and trimethoprim. Following the ban on incorporation of tetracycline in animal feeds for nutritive purposes, tetracycline resistance in S. typhimurium and S. panama strains of porcine origin dropped from about 90% in 1974 for both species, to about 34% and 1%, respectively, in 1980. The incidence of resistance in human strains concurrently decreased from about 80% in 1974 to 25% and 1%, respectively, in 1980.The build-up of multiple resistance in bovine S. dublin and S. typhimurium strains, already started in 1973–1974, has continued. Recently, phage type 193 S. typhimurium strains have become predominant and they are invariably resistant to ampicillin, chloramphenicol, tetracycline, kanamycin, neomycin, streptomycin, sulphonamide and trimethoprim. Up to now, type 193 strains were hardly encountered in human patients, but the number of human isolates is slowly increasing.A fairly large number of multiply resistant strains belonging to S. oranienburg, S. schwarzengrund, S. typhimurium and, recently, S. krefeld have been isolated from adoptive children from the Far East.     

Occurrence of Chloramphenicol-acetylating Enzymes in Various Gram-negative Bacilli

The occurrence of a chloramphenicol-acetylating enzyme, similar to that found in Escherichia coli, carrying an R factor was investigated in various gram-negative bacilli. The acetylated products of chloramphenicol were identified by chromatography and quantitatively assayed after benzene extraction. The investigated strains were of the Salmonella-Arizona group, the Klebsiella-Aerobacter group, Serratia marcescens, the Proteus group, and Pseudomonas aeruginosa, most of which were isolated from 1947 to 1957. Both chloramphenicol-sensitive and -resistant strains were included, but none of them was able to transfer chloramphenicol resistance by conjugation. In the Proteus group, a significant level of a chloramphenicol-acetylating enzyme was found in most strains, whether they were sensitive or resistant to chloramphenicol; the resistant strains showed higher levels of the enzyme. Some chloramphenicol-sensitive strains lacked this enzyme. Only the sensitive strains containing the enzyme could easily produce chloramphenicol-resistant mutants with higher enzyme activity. Thus, the chloramphenicol resistance of this group can be reasonably explained on the basis of the chloramphenicol-acetylating enzyme. All of the Pseudomonas aeruginosa strains were resistant to chloramphenicol, and most strains showed low levels of the enzyme (which, however, did not appear sufficient to explain their resistance). All of the strains of the other groups (except one strain of Enterobacter cloacae) lacked the enzyme, although most strains of the Klebsiella-Aerobacter group and of S. marcescens were resistant to chloramphenicol. With respect to the origin of the resistance gene of the R factor, it is noteworthy that the strains of Proteus mirabilis isolated in 1947 possessed this enzyme before the discovery of chloramphenicol.     

Specificity and mechanism of tetracycline resistance in a multiple drug resistant strain of Escherichia coli

Izaki, Kazuo (University of Tokyo, Tokyo, Japan), Kan Kiuchi, and Kei Arima. Specificity and mechanism of tetracycline resistance in a multiple drug resistant strain of Escherichia coli. J. Bacteriol. 91:628-633. 1966.-A decrease in the uptake of tetracycline occurred concurrently with a rise in the level of resistance of a multiple drug resistant strain of Escherichia coli grown in the presence of tetracycline. Although the strain was also resistant to streptomycin and chloramphenicol, growth in the presence of these two antibiotics did not influence the uptake of tetracycline. The induction of resistance, or decreased uptake of tetracycline, was dependent on growth of the organism in the presence of the drug. Decreased uptake of tetracycline could not be induced in a sensitive strain of the same organism under conditions suitable for induction of the resistant strain. The decrease in accumulating power of the resistant organism cultured in the presence of tetracycline does not appear to be due to selection of a resistant strain from cultures containing both resistant and sensitive strains.     

Multiresistance of Staphylococcus xylosus and Staphylococcus equorum from Slovak Bryndza cheese

Staphylococcus xylosus, Staphylococcus equorum, and Staphylococcus epidermidis strains were isolated from Bryndza cheese and identified using PCR method. The antimicrobial susceptibility of these strains was assessed using disc diffusion method and broth microdilution method. The highest percentage of resistance was detected for ampicillin and oxacillin, and in contrary, isolates were susceptible or intermediate resistant to ciprofloxacin and chloramphenicol. Fourteen of the S. xylosus isolates (45 %) and eleven of the S. equorum isolates (41 %) exhibited multidrug resistance. None of the S. epidermidis isolate was multiresistant. The phenotypic resistance to oxacillin was verified by PCR amplification of the gene mecA.